1. Field of the Invention
The present invention relates to an optical disc for storing multimedia data which includes digital moving picture data, audio data and sub-picture data and to a reproduction device for such disc and especially to a technique for the achievement of dynamic switching of the reproduced content.
2. Related Art
In recent years, laser discs and video CDs have been developed as optical storage media for storing multimedia data which is a mixture of moving pictures, audio, sub-pictures and the like.
The following is a simplified explanation of the data format, used by conventional optical discs for moving picture data and audio data which are compressed according to MPEG standard, by which the data called "system streams" are formed.
FIG. 1 is a drawing for explaining the way in which compressed moving picture data and compressed audio data make up these combined data streams called system streams.
In the drawing, "video elementary stream" is a one-dimensional video data stream which has been compressed, with this video data stream being divided into packets which are several bytes long and these packets being numbered "video 1", "video 2" . . . in the drawing. Similarly, "audio elementary stream" is divided into packets which are several bytes long after the left and right channel data has been combined during compression, with these packets being numbered "audio 1", "audio 2" . . . in the drawing. These packets are interleaved in suitable order for the buffer size of the reproduction device and for the reproduction time so as to provide the system streams shown in the drawing.
The following is an explanation of the case when a video CD to be used for karaoke is produced using such system streams.
For such a disc, a premise is supposed where the same background video data is used while one out of "duet", "male part" or "female part" is reproduced. In this case, "duet" requires a system stream in which background video data, to which subtitles for the lyrics of both the male and female parts of the vocal have been added, and audio data, which is only for the accompaniment, are interleaved. "Male part" requires a system stream in which background video data, to which subtitles for the lyrics of only the male part of the vocal have been added, and audio data, which is for the accompaniment and includes a female vocal for the female part, are interleaved. As can be imagined, the "female part" is the opposite of the male part.
For a video CD, it is necessary for these three data compositions to be stored as three different system streams. This is to say, it is necessary for the common background video data to be stored in each of the three system streams.
FIG. 2 shows the system streams in the above situation. In this drawing, system stream 1 is for a duet and so is made up of an interleaving of the background video data, the audio data for the accompaniment and sub-picture data which shows the lyrics for both the male and female parts. System stream 2 is for the male part and so is made up of an interleaving of the background video data, the audio data for the female vocal as well as the accompaniment and sub-picture data which shows the lyrics for only the male part. System stream 3 is for the female part and so is made up of an interleaving of the background video data, the audio data for the male vocal as well as the accompaniment and sub-picture data which shows the lyrics for only the female part. Also, a menu system stream is used to display a menu for selecting one of duet, male part and female part, as well as storing a menu information table which stores information for which system stream is to be reproduced when each of the reproduction system streams (duet, male part, female part) has been selected. In this way, three separate system streams are stored on the video CD corresponding to duet, male part and female part.
However, for conventional video CDs and video CD reproduction of devices, there has been the problem that after the reproduction a system stream for any of the "duet", the "male part" or the "female part" has started, it is not possible to change to a different part without stopping the flow of the reproduction, which is to say, without stopping the video data.
More specifically, for the same example of a karaoke video CD, it has not been possible for a user who wishes to switch to "duet" during the reproduction of the "female part" audio part to switch to the "duet" audio part. This is because the songs corresponding to "duet", "male part" and "female part" are expressed using separate system streams. This is to say, the menu information table can only be used at the start of reproduction of a system stream, so that it is not possible to have reproduction started at an address in the "duet" system stream which is synchronized to a current reproduction address in the "female part" system stream.
Also, for such a video CD, it is necessary to store the main video data which is common to different system streams in each system stream, which reduces the storage efficiency of the disc.